Tool posture control system

ABSTRACT

A tool posture control system according to the present invention rotates a tool vector v 1  indicative of a present tool position to a target tool vector v 1  through vector rotation, and generates data for driving robot axes from the data produced by the vector rotation. For a welding robot, for example, the inclination of a torch with respect to an arc starting surface can automatically and appropriately be established, so that an arc starting point can accurately be searched for. Therefore, the operation to search for a desired arc starting point can properly be carried out without human intervention.

DESCRIPTION BACKGROUND OF THE INVENTION

The present invention relates to a tool posture control system, and moreparticularly to a system for controlling the posture of a tool whilesearching for an arc starting point by controlling the posture vector ofthe searching posture of the tool using a tool coordinate system, and bytilting the tool in a certain direction in a searching coordinate spacefor thereby seaching for the arc starting point.

Industrial robots have found widespread use and are being used in manyfields in recent years. In welding technology, welding operations aregetting too difficult for human beings to perform, and the workingenvironments such as for automobile production lines are more likely tobe hard on workers. For these reasons, human operations are beingreplaced with welding robots.

FIG. 4 of the accompanying drawings shows a general welding robot. Theillustrated welding robot is an articulated robot having six axes. Thesesix axes include a T(θ)-axis about which an arm assembly rotates, aW-axis about which a lower arm is tilted back and forth, a U-axis aboutwhich an upper arm is tilted vertically, an A-axis about which a wristrotates in a horizontal plane, a B-axis about which the wrist moves in avertical plane, and a C(r)-axis about which the wrist rolls, these axesbeing independently controlled. Designated at 1 in FIG. 4 is a base onwhich the articulated robot is supported. A T(θ)-axis servo motor 2 ismounted in the base 1 for turning the axes about the vertical axis(Z-axis). On the T(θ)-axis servo motor 2, there is mounted a T(θ)-axisunit 3 rotated by the servo motor 2. A W-axis unit 4 is fixedly mountedon the T(θ)-axis unit 3, and a W-axis arm 5 is rotatably supported by apivot shaft 5a on the W-axis unit 4, the W-axis arm 5 being operated bya W-axis drive mechanism 6. A U-axis arm 7 is rotatably supported by apivot shaft 7a on the distal end of the W-axis arm 5, the U-axis arm 7being operated by a U-axis drive mechanism 8. A wrist mechanism 9 ismounted on the distal end of the U-axis arm 7. The wrist mechanism 9 isrotated by an A-axis servo motor 10, vertically swung by a B-axis servomotor 11, and rolled by a C-axis servo motor 12. Robot operation isperformed by a tool attached to the wrist mechanism 9. A torch used assuch a tool and an arc welding process employing the torch will bedescribed. FIG. 5 schematically shows an arc welding machine. A wire WRis fed by rollers FR in small increments in the direction of the arrow,and passes through a guide member GB to project from the distal end of atorch TC. The rate of feed of the wire WR is limited such that thedistal end of the wire will be spaced a prescribed distance from thesurface of a member WK to be welded. The positive potential of a highvoltage which is generated by a welding power supply PS intermittentlywith a given period is applied to the wire WR through the guide memberGB, whereas a negative potential is impressed on the member WK to bewelded. A gas is supplied from a gas supply (not shown) in the directionof the arrows through the torch TC and applied to the member WK toprevent a welded area from being oxidized. When the gas is supplied fromthe gas supply and the high voltage is intermittently generated by thewelding power supply PS while the wire is fed out in small increments,an arc is produced from the distal end of the wire, and the wire and themember to be welded are melted such that the melted portion isintegrally welded. Such a welding operation is performed by the robot.More specifically, the torch of the welding machine is gripped by therobot, and the torch (distal end of the wire) is moved by the robotalong a welding path to weld the desired portion.

When the member to be welded is set in place for the welding operation,the torch is moved with respect to the member to be welded to search foran arc starting point while manually searching for the position of themember to be welded.

For determining the tool position, there are employed a robot referencecoordinate system, a robot hand coordinate system, and a tool coordinatesystem. FIG. 6 is a diagram explanatory of such robot, hand, and toolcoordinate systems. The robot reference coordinate system is indicatedby x, y, z with O denoting the origin. Designated at l, m, n are handposture vectors in the robot hand coordinate system, while designated att, u, v are tool posture vectors in the tool coordinate system. Denotedat TCP is a tool center point (also referred to as a tool grip point).

Prior to starting arc welding, as described above, it is necessary tosearch for an arc-welding starting point. The arc starting point cannotaccurately be spotted and the desired operation to search for the arcstarting point cannot be effected unless the posture of the torch isappropriately controlled.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a tool posturecontrol system capable of desirably searching for an arc starting pointby automatically establishing the best posture of a working member tosearch for the position of a member to be welded.

A tool posture control system according to the present invention has arobot having an arm, a tool mounted on the distal end of the arm,principal vector setting means for determining the principal position ofthe robot, means for determining a tool vector indicative of the presentposition of the tool, means for determining a target tool vectorinclined with respect to the principal vector, rotating means forrotating the tool vector indicative of the present position of the toolto the target tool vector through vector rotation, means for generatingdata for driving the axes of the robot from data produced by rotatingthe tool vector indicating the present position with the rotating means,and for moving the tool to the position of the target tool vector.

Where the present invention thus arranged as above is applied to awelding robot, for example, the torch as the tool can automatically beset to a position having the best inclination when it is initiallyestablished. Since an arc starting point can thus be searched foraccurately, the desired operation for searching for the arc startingpoint can properly be performed without human intervention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the manner in which a tool posture vectoris controlled in a searching coordinate space;

FIG. 2 is a flowchart of operation of means for controlling the toolposture vector;

FIG. 3 is a view showing an example to which a process of controllingthe tool posture vector is applied;

FIG. 4 is a view of a general welding robot;

FIG. 5 is a schematic view of an arc welding machine; and

FIG. 6 is a diagram explaining a robot coordinate system and a toolcoordinate system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be described inspecific detail with reference to the drawings.

FIG. 1 is explanatory of the control of the tool posture vector of atorch in a searching coordinate space, i.e., a coordinate space with amember to be welded being used as a reference, where the underlining ofa letter indicates a vector. FIG. 2 is a flowchart of the controlsequence. Principal vectors in the searching coordinate space areindicated as d (x direction), a (y direction), and h (-z direction).Designated at t, u, v principal vectors in the tool coordinate space.

Searching posture directions for the tool are determined by anglesbetween the principal vectors d, a, h in the searching coordinate spaceand the three fundamental t, u, v.

Since the searching directions are fixed, the tool posture vectors aresimplified, and the searching directions are easy to compute.

A process of controlling the searching posture of the tool at the timeof searching for an arc starting point will be described below.

A present tool vector v, is first determined.

In order to transform the tool vector v₁ into the searching coordinatespace, a target tool vector v' is established with respect to each ofthe principal vectors d, a, h in the searching coordinate space.Specifically,

(1) The target tool vector v' is determined with respect to the vector-d which is one of the principal vectors in the searching coordinatespace. This tool vector v' is inclined θ from the principal vector -d,the angle θ being preset to an optimum value.

(2) Then, the target tool vector is determined with respect to thevector a that is one of the principal vectors in the searchingcoordinate space.

(3) Thereafter, the target tool vector is determined with respect to thevector h that is one of the principal vectors in the searchingcoordinate space.

When transforming the present tool vector v₁ into the searchingcoordinate system, the present tool vector v₁ is rotated to the targettool vector v'. This is expressed by:

    v'+Rot(R, Θv)v.sub.1

where Rot indicates rotation, R is the center of rotation, and θv is theangle of rotation.

The present tool vector v₁ has thus been transformed into the searchingcoordinate system, and a tool posture vector is now obtained.

The transformed tool posture vector is then transformed to a robot handposture vector, i.e., can be transformed to robot axis data. Morespecifically, normal and inverse transformation can be possible at alltimes between the hand posture vectors l, m, n and the fundamental axest, u, v in the tool coordinate system. This is because the handcoordinate system and the tool coordinate system are always in fixedrelative spatial positions, and hence the following equations can beestablished through one 3×3 fixed matrix (M): ##EQU1##

To determine (M), metric values for the axes of the robot are used assetting data, and hand posture vectors lo, mo, no at this time can bederived from these values. Assuming that

    lo, mo, no=T.sub.(M)

transformation can easily be performed between the hand posture vectorsl, m, n and the fundamental axes t, u, v in the tool coordinate system.In this manner, the torch serving as the tool can approach a givensearching surface at a desired angle, with the result that an accuratearc starting point can be searched for.

Actual control of the searching posture of a tool with respect to anL-shaped member to be welded, using the process of controlling theposture of the torch as the tool, will be described below.

FIG. 3 is a view illustrative of such control of the searching postureof the torch.

It is assumed that the torch (distal end of the wire) TC as the tool islocated at the position ○1 and inclined at any angle. Theabove-described tool posture control is effected in this position tocause the torch TC to be inclined at θ₁ with respect to a lineperpendicular to a side surface ○a . From this position , the torch TCmoves in a searching manner toward the side surface ○a of a verticalplate of the L-shaped member until the torch TC contacts the sidesurface a at a position ○2 . Then, the torch TC returns to the initialposition ○1 , wherein the torch posture control is carried out to directthe torch TC to be inclined at θ₂ with respect to the upper surface ○bof a horizontal plate of the L-shaped member. Thereafter, the torch TCis lowered until it contacts the upper surface b at a position ○3 . Inthe position ○3 , the torch TC is controlled in its posture again so tobe inclined at θ₃, and then the torch TC is moved in a searching mannertoward a side edge of the horizontal plate of the L-shaped member. Whenthe torch TC reaches the edge ○4 , the foregoing torch posture controlis performed again to move the torch TC toward an arc starting point S.

It should be understood that the present invention is not limited to theabove embodiment, but many changes and modifications may be made thereinwithout departing from the scope of the present invention.

The present invention is not limited to welding robot applications, butmay suitably be employed for controlling the posture of otherarticulated robots.

What is claimed is:
 1. A tool posture control system, comprising:a robothaving an arm; a tool mounted on the distal end of the arm; principalvector setting means for determining a position of the robot in a robotcoordinate system; means for determining a tool vector indicative of thepresent position of the tool in a tool coordinate system; means fordetermining a target tool vector inclined with respect to a principalvector in a searching coordinate system; rotating means for rotating thetool vector indicative of the present position of the tool to the targettool vector through vector rotation; and means for generating data fordriving the axes of the robot from data produced by rotating the toolvector indicating the present position with said rotating means to movethe tool to the target vector position by transforming the tool vectorfrom the tool coordinate system, to the searching coordinate system andto the robot coordinate system.
 2. A tool posture control systemaccording to claim 1, wherein said tool comprises a welding torch of awelding robot.
 3. A tool posture control system according to claim 1,wherein said robot comprises an articulated robot having a plurality ofarticulations.